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SolutionsReactivity

1. Homolytic or heterolytic

State whether each cleavage is homolytic or heterolytic:

  1. CH4 → CH3· + H·
  2. HCl → H+ + Cl−
  3. Cl2 → 2 Cl·
  4. H2O → H+ + HO−
  1. Homolytic: each atom keeps one electron and radicals are formed.
  2. Heterolytic: chlorine, more electronegative, keeps both electrons.
  3. Homolytic.
  4. Heterolytic.

2. Why are they so unreactive?

The C–H bonds of propane (98 kcal/mol) are no stronger than the N–H bond of ammonia (107 kcal/mol). Why, then, are alkanes so unreactive?

Because C–H and C–C bonds are practically nonpolar (C and H have similar electronegativities) and alkanes have no lone pairs and no π bonds. There are no electron-rich or electron-poor regions to attract other reagents, so they only react by homolytic cleavage, which is costly.

3. The four typical reactions

Name the typical alkane reaction that corresponds to each transformation:

  1. CH3CH2CH2CH3 → (CH3)3CH
  2. CH4 + Cl2 → CH3Cl + HCl
  3. CH3CH3 → CH2=CH2 + H2
  4. CH4 + 2 O2 → CO2 + 2 H2O
  1. Isomerization.
  2. Halogenation.
  3. Dehydrogenation.
  4. Combustion.

4. Radicals and ions

Which species are formed in the homolytic cleavage of a bond? And in heterolytic cleavage?

Homolytic cleavage gives two free radicals, each with an unpaired electron. Heterolytic cleavage gives a cation and an anion.

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